After running through the mass numbers for Skylon that Reaction Engines offers, I'm left believing that they've got an engine that will theoretically get them to orbit in one stage, but only by them making unrealistic assumptions about the structural weight of the rest of the vehicle, total dry mass 53 tonnes, engine thrust 270 tonnes, engine T/W 14 therefore mass of engines ~19 tonnes, therefore mass of the rest of the vehicle ~34 tonnes. 34 tonnes for a winged vehicle that's 83 meters long, carries all its landing gear to orbit, it has a propellant volume around 1400^3 meters, it uses cryogenic propellants and it has to endure re-entry. I know they're promoting Skylon as having revolutionary construction materials and methods, but it seems to me they've had to make some excessively optimistic assumptions about the structural weight to get the numbers to come together so they can continue with their pet project - the engines.
Looking at it another way: The combined propellant tank volume by my math (with a few assumptions on the current LH2:LOX ratio) would have enough volume to hold 500 tonnes of LH2/LOX at a 1:6 ratio, lets allow structural weight growth of 20% for the heavier take-off weight making structural wt 40.8 tonnes, 2 SSME's (or easily maintained equivalent) is + 6.4 tonnes, so total unladen weight is 47.2 tonnes, add a P/L of 15 tonnes and also the 500 tonnes LOX/LH2 and you get a take-off weight of 562.2 tonnes, at engine shut off weight is 62.2. Mo/M1 is 9.03, delta V at Ve 4500 m/s is 9907m/s.
9.2 km/s is about all you need to get into orbit.
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